hi,
I have the next wall build up:
Plasterboard
38mm Service void, formed with battens
OSB3 as airtightness layer
300mm ‘I’ Beams filled with Warmcell insulation
16 mm Agepan DWD wind tight, exterior protection board.
40mm ventilated cavity (1mm moisture capacity-38mm no moisture capacity- 1mmm moisture capacity)
20mm timber cladding
would you recommend to add a rain source in this build up? if yes, where?
Should I carry out the water content limit procedure check for this wall build up (Guideline for assessing the risk of interstitial condensation runoff)?
Do I need take into account the drying of construction moisture (phase 1 + phase 2)? In the guideline (https://wufi.de/download/guideline/Guid ... 240716.pdf) it is a masonry wall but I have a lightweight timber frame construction. if not, what initial conditions do you recommend for the materials?
Best regards,
Rain source location
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Alberto Morales
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Christian Bludau
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Re: Rain source location
Hi Alberto,
I would primarily use a rain source if there is a realistic possibility that rainwater could penetrate behind the DWD board, for example at window connections, penetrations, or other vulnerable details. Lightweight timber-frame constructions are often much less tolerant of rainwater intrusion than massive constructions. Therefore, ensuring a rain-tight design and proper execution of all weatherproofing details is particularly important. A rain source can be useful to assess the consequences of potential leakage and the robustness of the assembly.
The water content limit procedure described in the guideline is primarily intended to assess the risk of moisture accumulation and runoff within the construction. For a timber frame wall, it can be a useful additional check if significant interstitial condensation is predicted. However, in many cases the standard evaluation criteria (moisture content, relative humidity, mold growth risk, and long-term drying behavior) are the primary indicators. Whether the runoff assessment is necessary depends on the results of the initial simulation.
Regarding construction moisture, the situation is different from masonry constructions. The two-phase approach in the guideline mainly addresses wet construction materials that require a prolonged drying period. In a lightweight timber frame assembly, construction moisture is usually much lower unless the materials have been exposed to rain during construction or were installed with elevated moisture contents.
Regarding the initial conditions, for lightweight timber-frame constructions it is common practice to start with an initial climate of 20 °C and 80 % RH. This assumption represents a moderate level of construction moisture and has proven to be a practical starting point for hygrothermal assessments of timber-frame assemblies. Additional construction moisture usually does not need to be considered unless there is evidence that the materials were exposed to significant wetting during storage or construction. In such cases, a separate sensitivity analysis with elevated initial moisture contents may be appropriate.
Christian
I would primarily use a rain source if there is a realistic possibility that rainwater could penetrate behind the DWD board, for example at window connections, penetrations, or other vulnerable details. Lightweight timber-frame constructions are often much less tolerant of rainwater intrusion than massive constructions. Therefore, ensuring a rain-tight design and proper execution of all weatherproofing details is particularly important. A rain source can be useful to assess the consequences of potential leakage and the robustness of the assembly.
The water content limit procedure described in the guideline is primarily intended to assess the risk of moisture accumulation and runoff within the construction. For a timber frame wall, it can be a useful additional check if significant interstitial condensation is predicted. However, in many cases the standard evaluation criteria (moisture content, relative humidity, mold growth risk, and long-term drying behavior) are the primary indicators. Whether the runoff assessment is necessary depends on the results of the initial simulation.
Regarding construction moisture, the situation is different from masonry constructions. The two-phase approach in the guideline mainly addresses wet construction materials that require a prolonged drying period. In a lightweight timber frame assembly, construction moisture is usually much lower unless the materials have been exposed to rain during construction or were installed with elevated moisture contents.
Regarding the initial conditions, for lightweight timber-frame constructions it is common practice to start with an initial climate of 20 °C and 80 % RH. This assumption represents a moderate level of construction moisture and has proven to be a practical starting point for hygrothermal assessments of timber-frame assemblies. Additional construction moisture usually does not need to be considered unless there is evidence that the materials were exposed to significant wetting during storage or construction. In such cases, a separate sensitivity analysis with elevated initial moisture contents may be appropriate.
Christian
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Alberto Morales
- WUFI User
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Re: Rain source location
thanks for the reply, but, is it correct to follow "Guideline for the Calculation and Evaluation of an ETICS with Wood Fibre Insulation" for the build-up assembly that I have?
Plasterboard
38mm Service void, formed with battens
OSB3 as airtightness layer
300mm ‘I’ Beams filled with Warmcell insulation
16 mm Agepan DWD wind tight, exterior protection board.
40mm ventilated cavity (1mm moisture capacity-38mm no moisture capacity- 1mmm moisture capacity)
20mm timber cladding
Plasterboard
38mm Service void, formed with battens
OSB3 as airtightness layer
300mm ‘I’ Beams filled with Warmcell insulation
16 mm Agepan DWD wind tight, exterior protection board.
40mm ventilated cavity (1mm moisture capacity-38mm no moisture capacity- 1mmm moisture capacity)
20mm timber cladding
Last edited by Alberto Morales on Fri Jul 31, 2026 9:45 pm -1100, edited 2 times in total.
x
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Christian Bludau
- WUFI SupportTeam IBP
- Posts: 1300
- Joined: Tue Jul 04, 2006 10:08 pm -1100
- Location: IBP Holzkirchen, the home of WUFI
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Re: Rain source location
I would be cautious about directly applying the "Guideline for the Calculation and Evaluation of an ETICS with Wood Fibre Insulation" to your assembly.
The guideline was developed specifically for ETICS systems with wood fibre insulation. Your wall build-up differs significantly, particularly because the cavity between the I-joists is filled with Warmcel (cellulose insulation). Cellulose has different moisture storage and moisture redistribution characteristics compared to wood fibre insulation, so the assessment criteria are not necessarily transferable.
That said, some of the general hygrothermal principles discussed in the guideline may still be useful. However, I would recommend evaluating the results in the context of the properties of cellulose insulation rather than relying solely on the acceptance criteria established for wood fibre ETICS systems.
The picture is from the pitched roof guideline an not related to your ETICS?
Christian
The guideline was developed specifically for ETICS systems with wood fibre insulation. Your wall build-up differs significantly, particularly because the cavity between the I-joists is filled with Warmcel (cellulose insulation). Cellulose has different moisture storage and moisture redistribution characteristics compared to wood fibre insulation, so the assessment criteria are not necessarily transferable.
That said, some of the general hygrothermal principles discussed in the guideline may still be useful. However, I would recommend evaluating the results in the context of the properties of cellulose insulation rather than relying solely on the acceptance criteria established for wood fibre ETICS systems.
The picture is from the pitched roof guideline an not related to your ETICS?
Christian
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Alberto Morales
- WUFI User
- Posts: 83
- Joined: Tue Mar 29, 2022 10:57 pm -1100
Re: Rain source location
thanks for the reply.
the image is an error so my bad...you can delete it to avoid confusion for people reading the post.
My wall has got cellulose between I joist + woodfibre to the exterior so, does the guideline apply just for ETICS systems with wood fibre insulation?
the image is an error so my bad...you can delete it to avoid confusion for people reading the post.
My wall has got cellulose between I joist + woodfibre to the exterior so, does the guideline apply just for ETICS systems with wood fibre insulation?
x
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Christian Bludau
- WUFI SupportTeam IBP
- Posts: 1300
- Joined: Tue Jul 04, 2006 10:08 pm -1100
- Location: IBP Holzkirchen, the home of WUFI
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Re: Rain source location
Hi Alberto,
The guideline was primarily developed for standard ETICS systems with wood fibre insulation applied to typically mineral wall constructions.
As stated in the introduction:
That does not necessarily mean that the recommendations are not useful in your case. Many of the underlying hygrothermal principles are still relevant. However, the moisture behaviour of lightweight timber constructions can differ significantly from that of masonry walls with ETICS, and additional assessments may be required.
In addition, the guideline primarily addresses the drying of construction moisture from the wall into the external thermal insulation composite system (ETICS). This is a key aspect of the assessment methodology described in the document. In lightweight timber-frame constructions with cellulose insulation between I-joists, this mechanism is generally not relevant, as cellulose insulation is installed dry and does not introduce significant amounts of construction moisture into the assembly. For this reason, one of the central assumptions underlying the guideline is not applicable to your construction, and the guideline is therefore of limited suitability for assessing this type of wall assembly.
For your wall assembly, I would therefore use the guideline with caution and not assume that all recommendations and assessment criteria can be transferred directly without further verification.
Christian
The guideline was primarily developed for standard ETICS systems with wood fibre insulation applied to typically mineral wall constructions.
As stated in the introduction:
Therefore, it is not intended specifically for lightweight timber-frame constructions with cellulose insulation between I-joists and an external wood fibre sheathing layer.This guideline provides support for the evaluation of wood fibre ETICS from manufacturers represented in the Verband Dämmstoffe aus nachwachsenden Rohstoffen e.V. (vdnr) using transient hygrothermal simulations.
That does not necessarily mean that the recommendations are not useful in your case. Many of the underlying hygrothermal principles are still relevant. However, the moisture behaviour of lightweight timber constructions can differ significantly from that of masonry walls with ETICS, and additional assessments may be required.
In addition, the guideline primarily addresses the drying of construction moisture from the wall into the external thermal insulation composite system (ETICS). This is a key aspect of the assessment methodology described in the document. In lightweight timber-frame constructions with cellulose insulation between I-joists, this mechanism is generally not relevant, as cellulose insulation is installed dry and does not introduce significant amounts of construction moisture into the assembly. For this reason, one of the central assumptions underlying the guideline is not applicable to your construction, and the guideline is therefore of limited suitability for assessing this type of wall assembly.
For your wall assembly, I would therefore use the guideline with caution and not assume that all recommendations and assessment criteria can be transferred directly without further verification.
Christian